But the best-case scenario, that the deal would exert a moderating influence on Iran’s behavior, has yet to be realized. Human rights abuses have piled up and Tehran has conducted missile tests that U.N. chief Ban Ki-moon has called inconsistent with “the constructive spirit” of the deal.

Still, the pact between Iran and six world powers has achieved what Secretary of State John F. Kerry and his negotiators set out to do — cramp Iran’s ability to build nuclear weapons.

Within six months, Iran pared down its uranium and heavy-water stockpiles and poured cement in the core of its Arak reactor. It has allowed unprecedented access to monitoring by the International Atomic Energy Agency (IAEA). Washington and Tehran now communicate directly, even without official diplomatic ties.

But the Joint Comprehensive Plan of Action is under threat from the half of the deal that kicked in six months ago, when the United States, the European Union and the United Nations revoked nuclear-related sanctions.

European businesses have been leery of running afoul of U.S. sanctions over human rights abuses and terrorism, and Tehran has complained the United States is not doing enough to let them know what is legal. Kerry has gone out of his way to outline the new rules, to little avail and much criticism from the deal’s detractors. U.S. officials say they are upholding their end of the deal.

“The United States has bent over backwards trying to ensure Iran is getting the sanctions relief it deserves,” said Robert Einhorn, an arms-control expert at the Brookings Institution. “There’s a risk the JCPOA will become a scapegoat for Iranian critics of the deal. The U.S. administration very much wants see Iran get the benefits of the deal, for no other reason than it wants to see Iran meet its nuclear commitments.”

In Washington and Tehran, the agreement remains controversial. Congress has tried to impose new limits. In Iran, hard-liners have conducted ballistic-missile tests and arrested dual nationals in defiant display of their influence.

“There are those of us who are concerned that unless there’s a change in Iran’s other egregious behavior, its nascent nuclear status is problematic,” said Mark Wallace, head of United Against Nuclear Iran, which opposed the deal and now is warning corporations of potential pitfalls.

The agreement finalized in Vienna has also won praise.

“As a result of the JCPOA, all pathways to an Iranian nuclear weapon have been blocked, thereby providing greater security to our friends and partners in the region and to the world,” said an open letter released Tuesday, signed by 75 leading scientists, generals, former ambassadors and members of Congress.

Much of what is known about what Iran is doing comes from IAEA inspectors who monitor Iran’s commercial nuclear plants, as well as its uranium mines and factories.

The IAEA sends reports to Mull and his counterparts from Britain, France, China, Russia and Germany, who all meet with Iranian officials every three months in Vienna. Mull said he is in daily contact with the IAEA and exchanges emails at least twice a week with Iranian officials from the country’s foreign ministry or central bank.

“There’s not a lot of trust in this relationship,” he said. “So whenever there’s an indication that Iran is not complying with what it committed to do, that’s always something that gets a lot of attention in Washington.”

This spring, the IAEA reported Iran had slightly exceeded its permissible stockpile of heavy water. Mull said he emailed Tehran asking what was going on. Iran quickly acknowledged it was over the limit and shipped out 20 tons within 48 hours. Eventually, the United States agreed to buy it for $8.6 million to be resold commercially.

“With that one exception, which was corrected so quickly we didn’t consider it a violation, they have been in compliance with every other limit,” Mull said.

Some worry the IAEA, and therefore the United States, is not catching everything. A German intelligence report leaked recently said Iran in 2015 attempted clandestinely to acquire dual-use technology that can be used for both civil and military purposes. But its shopping spree was before the deal’s January “Implementation Day.”

David Albright, a nuclear expert with the Institute for Science and International Security, fears Iran could still be trying to get its hands on prohibited material and technology.

“We don’t know what we don’t know,” he said. “The deal has been a real success in getting deep reductions in Iran’s nuclear capability. But now Iran is trying to push back and get new capabilities.”

Iran’s missile tests in particular have raised concerns in Washington, though administration officials argue they would be more threatening if Iran possessed nuclear weapons. Iran has never recognized the legitimacy of U.N. missile sanctions, and the deal’s wording softened the language from an explicit ban to a suggestion.

Mark Dubowitz, head of the Foundation for the Defense of Democracies and a vocal critic of the agreement, said Iran should be sanctioned further.

“I’m a little surprised the Iranians have been so bold in the first year after the agreement, in significantly ramping up their regional aggression, regional terrorism, missile testing and human rights abuses,” he said. “I thought they would have played nice for a couple years.”

Many of the agreement’s supporters say the deal had a singular purpose.

“We signed the agreement because some people in Iran wanted to get nuclear weapons, so it was good to head that off or make it harder ,”said Jeffrey Lewis of the James Martin Center for Nonproliferation Issues. “But the idea it would lead to a transformation in our relationship and inside Iran always seemed unrealistic to me.”

The next year could be fateful for the agreement. The United States will have a new president in January. Iranian President Hassan Rouhani, a pragmatist who pushed for the negotiations, faces reelection next year. The outcome may rest on whether Iranians feel the deal’s economic benefits.

“I’m not panicked it’s going to fall apart,” said Richard Nephew, who headed the U.S. sanctions team in negotiations until early last year. “But if we don’t see things start to improve inside Iran, it will be much more difficult for Rouhani to stay on course.”

Mull sometimes steps back and marvels at what has been accomplished.

“I know there are people who have different views of Iran and this deal in general,” he said. “But the fact is, six months after the deal was implemented, it’s working and has reduced the threat of a nuclear Iran. Every day we have achieved that, it’s good for our friends and our country.”